File.mtime() is slower than it should be

This is a benchmark result for File.mtime on my system:
1.268000 0.000000 1.268000 ( 1.268000)

And this is File.ctime:
0.716000 0.000000 0.716000 ( 0.716000)

There's no reason mtime should be slower than ctime, since they can both be discerned from the output of File.stat. JRuby currently doesn't use stat() for File.mtime, but an alternate path (with a comment about jna being too slow ...)